1 64 GARDINER. [Vol. XI. 



through E, and E'l.r.i descended from A' through E'. All cells 

 marked by letters bearing the sign ' lie on the right of the 

 shorter axis as viewed in the figures, except B' and its descen- 

 dants. The cells B and B' originated from A and A' respectively, 

 and when first formed B was to the left and B' to the right 

 (Fig. 2), and though in a later stage they have come to lie 

 directly on the shorter axis, the letter B is still applied to the 

 cell which was budded off from A, and B' to the cell budded 

 off from A'. 



The cell B divides subsequently into Bi and Bz. B\ will 

 divide into B\.\ and B1.2, and B2 will divide into Bz.x and B2.2. 

 The derivatives from B^ on the opposite side of the ovum will be 

 numbered similarly, but all marked with the sign '. The cells 

 C, D, E, C, D', and E' are, however, so placed that in the 

 subsequent cleavage some cells derived from each of them will 

 lie on the right and some on the left of a line drawn through 

 the long axis of the ovum. The cells lying on the right will be 

 distinguished by an r, and those to the left by an /; thus Cr 

 and CI, Dr and Dl (Fig. 13). As these cells further divide, 

 those derived from them will be indicated by numbers which 

 will show the generation to which they belong ; thus, Z>r divides 

 into Dr\ and Drz. In the next generation Dri gives rise to 

 Dr\.\ and Dri.z ; while Drz divides into Drz.x and Drz.z. 



The system by which the cells arising from E and E' will 

 be distinguished, necessarily differs slightly from that applied 

 to B, C, D, and B', C, D' , for E and E do not give rise to cells 

 which lie to the right or left of the long axis till a somewhat 

 later stage. In the first generation these cells give rise to 

 cells E\, E2, Et^, E4, and E'l, E 2, E t^, E\, some of which lie 

 directly on the line of the longer axis which will divide the 

 ovum into right and left halves (Figs. 14, 15 and 16). In the 

 next generation, however, when certain of these cells divide 

 into right and left, they will be distinguished by the letters r 

 and /; thus ^srand E2I (Fig. 18). 



To return now to the process of segmentation when the ten- 

 celled stage has been reached. As a rule the next cells to divide 

 are B and B', although it often happens that their division is 

 delayed till later. In either case, whether they divide now or 



